Late years, along with popularization of displays, a function required for an optical film for use therein has become increasingly diversified. Therefore, there is a need for film developments based on a new material and a new process.
Apart from film capabilities, in order to cope with size enlargement/thickness reduction of a display and an increase in production volume of displays, it is required to produce a large-width and thin film in large numbers. In production of such a thin-layer film, film strength is a key design parameter, in terms of runnability, processability and product reliability.
With regard to runnability, it is usual to roll-convey a film under a contact with various rolls, so as to continuously produce a long film for large-scale production. This involves a problem that, when folding or crinkling occurs in the film, a force is applied in a width direction thereof, which is likely to cause the film to tear in a longitudinal direction thereof. On the other hand, if the film is excessively largely bent during conveyance along a roll, the film is likely to tear in the width direction. Further, after slitting an end face of the film, or when an impact force is applied to the end face, the film is also likely to tear in the width direction. As above, in order to allow a film to run in a stable manner, it is necessary to ensure sufficient film strength in both longitudinal and width direction thereof.
With regard to film processability, poor film strength leads to a problem that breaking, such as tearing, is likely to occur in a film due to, for example, force during an operation of cutting the film into a sheet shapes, bending during an operation of laminating the film to another film or a panel, or bending during an operation of reworking the film from a panel.
With regard to product reliability, under a high-temperature and high-humidity environment, or under conditions of rapid change in temperature and/or humidity, a crack is likely to occur in a film due to failure in following shrinkage and/or expansion of a polarizer or other laminated films, causing non-uniformity of image display.
For this reason, it is necessary to improve film brittleness in terms of runnability, processability and product reliability in order to produce a large-width and thin film in large numbers.
In particular, when mention is made of an organic EL display, a circularly polarizing plate having a λ/4 retardation plate is employed in the organic EL display, for an anti-reflection purpose. Further, a reverse wavelength dispersion-type retardation plate is required to neutralize a reflection color. In order to cope with a larger type having potential to become popular in future, while fulfilling high image quality and small thickness as features of the organic EL display, it is necessary to provide a reverse wavelength dispersion-type thinned λ/4 retardation platewhich is large in width. With a view to imparting a reverse dispersion property, for example, to a copolymer, if a weight ratio between a main chain and a side chain is adjusted in such a manner as to increase the ratio of the side chain, a film is liable to become brittle. Consequently, the film has poor stretchability, causing difficulty in conducting design for thickness reduction and/or size enlargement.
A retardation film is known as one of the optical films. As one example of the retardation film, there has been developed a type having a property that a retardation value becomes larger as measured using light with a longer wavelength (also referred to as “reverse wavelength dispersion property”), as disclosed, for example, in the following Patent Documents 1 and 2. However, in the Patent Documents 1 and 2, no discussion about improvement in the above film brittleness is made.